Manual Focusing with AF Camera Systems

Superb image quality in today’s world of photography is fostered by a whole host of different factors such as the high numbers of pixels in digital SLR cameras, large format sensors in full-frame 35mm format or traditional almost-medium format, intelligent image processing techniques and noise reduction algorithms.
The results are also dependent on how the camera interacts with the lens, which is why high-grade optics have such a vital role to play in fulfilling photographers’ increasingly sophisticated demands with regard to image quality.

The weakest link

Achieving top-quality shots with high-resolution cameras means keeping all the parameters that have a direct or indirect impact on image quality within strict boundaries. The idea that “a chain is only as strong as its weakest link” has particular relevance for photography and image reproduction.

Precision focusing

One of the key links in the chain of successful image rendering is the precision with which the lens focuses on the main subject. Generally speaking, a photographic lens only provides optimum rendering at maximum image quality of a two-dimensional plane. This plane runs exactly in parallel to the film or sensor in the camera. Depending on the magnification and the selected aperture, a certain range both in front of and behind the point of optimum focus is also considered to be "adequately sharp". In this context, the magnification refers to the ratio between the image produced by the lens and the object being photographed. Thus, the focal length of the lens, the shooting distance and the size of the film or sensor are responsible for the so-called depth of field. The focusing region designated as the depth of field is the extent of the range in the object space of an imaging optical system. This region is rendered in acceptably sharp focus on the focal plane.

You can read about the effect that slight defocusing has on image quality in the article "Measuring lenses objectively" which appears in Camera Lens News Nr. 30 starting on page 24. This article shows how important precise focusing is whenever a photographer intends to produce big enlargements or requires optimum quality for any other reason, and not only when using wide apertures.

What should I focus on?

The question of what main subject to focus on obviously comes down to the taste of each individual photographer. Nevertheless, there are not likely to be any significant differences in opinion when it comes to choosing the optimum focal point for reproductions of a painting or photographs of a mountain chain that stretches into the endless distance. Equally, traditional portrait photography continues to reserve maximum sharpness for the model's eyes.
Tinkering with sharp focus and blur is one of the most fundamental creative aspects of photography. The fact that our environment is three-dimensional means that people can have differing views on what should be depicted with clarity and what should appear in a sketchy or blurred form, for example when searching for the right focus in a group of people seated around a circular table under difficult lighting conditions. Checking the depth of field using the camera's preview button is one technique that can be useful. Good results can also be achieved by the use of face recognition software in modern cameras that focuses the lens on the nearest person, especially when using compact cameras with small sensors and short focal lengths. However, this is clearly inadequate for the purposes of carefully arranged photography using SLR or rangefinder cameras, where high apertures and longer focal lengths are the norm. In these situations, the photographer is still required to choose the required autofocus frame or select a subject using the manual focus function. The photographer's aim here is to create a powerful, unique image. Any small deviations in the focal plane could potentially diminish the technical achievement of the image or even completely alter the impact of the photo, whether deliberately or not.

Advantages of AF systems

Autofocus systems have undergone major developments since they were first introduced in cameras in the 1980s. Manufacturers have steadily improved their systems' performance and efficiency in daily use, and the combination of an increasing number of AF points in the camera bodies and ultrasonic motors in the lenses has facilitated fast and smooth focusing for the phase AF systems typically found in today's SLR cameras. For some applications, it is precisely this speed that marks the key benefit over other AF methods such as the contrast-based AF typically found in digital compact cameras or manual focusing.
Good AF systems can generally achieve a very high number of "hits" in terms of the sharp images they produce with long telephoto lenses in situations such as taking shots of cheetahs in the wild, snapping celebrities from within a pack of paparazzi or photographing footballers on the playing field.

So is there any place left for manual focusing?

For any subject that is either not actively moving away from the photographer's position or that is set to move into the “trap focus” after careful pre-focusing, better results can be achieved with careful manual focusing. Carefully framed landscape shots, images of buildings or architectural details and meticulously arranged tabletop displays in a photographer's studio are unlikely to require the use of autofocus.
Equally, both spontaneous portraits and reportage shots can achieve the same focusing precision by manual means as by using an AF system. Macro shots of butterflies on a flower that require a very shallow depth of field – where a tripod is generally a sensible choice – should always be precisely focused using manual methods. In these cases, it is rare for any of the camera's AF points to coincide exactly with the photographer's choice of focal point and, all too frequently, the focusing point covers too much of the subject with a large extension of depth, which means that it is no longer possible to use the automatic mechanism to achieve precise focus.

You can read about the effect that slight defocusing has on image quality in the article "Measuring lenses objectively" which appears in Camera Lens News Nr. 30 starting on page 24. This article shows how important precise focusing is whenever a photographer intends to produce big enlargements or requires optimum quality for any other reason, and not only when using wide apertures.

Were older cameras and lenses actually better?

Anyone who has attempted to manually focus modern AF SLR cameras and their corresponding AF lenses will have quickly come to the sobering conclusion that, practically speaking, this is far from easy. Steep-pitch helical mounts, play and backlash in the focus rings of the lenses, dim viewfinders and far-from-suitable screens in the cameras make it very difficult to achieve high-precision focusing. In contrast, it is quite a revelation to go back to one of the good SLR cameras with a suitable lens from the era before the widespread introduction of AF systems and witness the large, bright viewfinder, the eminently practical adjustment aids on the screen and the way in which a subject seems to "spring to life" when it is correctly in focus.
Few of today's cameras achieve the innate viewfinder quality of these analog models that have now entered into the annals of history. Nevertheless, some good digital camera models from the medium to top-class brackets, especially full-frame cameras, do offer some advantages for high-precision manual focusing, at least thanks to the glass prism in the viewfinder and corresponding viewfinder magnification.

Manual photographic lenses

Nowadays, almost all manufacturers only offer AF interchangeable lenses and some systems – such as the Canon EOS system – have never even featured manual lenses at all (apart from a few specialist models). The current range of Carl Zeiss SLR lenses incorporates some of the expertise we have acquired from our professional cine lenses. With large rotation angles (e.g. almost 360 degrees in the case of the Makro-Planar T* 2/100 from infinity to the close-up limit of 0.44 m), a wealth of available settings for the all-metal unit and none of the constraints caused by the need to turn a focus motor, these lenses achieve a level of focusing accuracy that is far superior to any conventional AF lenses. Thanks to the popular range of lens mounts comprising the ZF (Nikon F bayonet mount), ZE (Canon EF bayonet mount) and ZK (Pentax K bayonet mount), the lenses can be directly employed on suitable cameras at high levels of functionality without requiring an adapter.

AF cameras with MF lenses

This leads us to the question of how far this accuracy can also be achieved using modern (D)SLR cameras, most of which suffer from the limitations described above in comparison to "old" models without AF. The focusing screens fitted as standard to modern autofocus cameras are primarily designed for a bright, crystal-clear viewfinder. What they generally lack, however, are optical focusing aids (e.g. split image rangefinder and microprisms) and – due to their surface structure – the ability to make any viable distinction between "in focus" and "out of focus". In addition, AF focus points and other markings that appear either on a second screen positioned above or on an LCD display can make it hard for the eye to clearly check the focus on a single plane of the screen.

Other types of screen

By replacing the focusing screen with a variant that has been optimized for manual focus, it is possible to achieve some major improvements. This procedure can be carried out for all SLR camera models, even if the manufacturer claims otherwise and fails to offer any interchangeable focusing screens.
For example, Canon offers a choice between the EC-A (with microprism ring) or EC-B (with split image rangefinder) for the models from its EOS-1 range. Meanwhile, the EOS 5D and 5D MkII have the option of the Ee-S screen, though this does not feature any focusing aids and therefore offers no significant benefits over the screen fitted as standard.
Both these lenses and smaller models such as the Canon EOS 50D and 40D, Nikon models up to the D700 and DSLRs from Pentax and Fuji can be fitted with excellent focusing screens produced by other manufacturers who offer variants that have been specifically tailored to each specific model. By following the manufacturer's instructions and, in some cases, using the supplied tool, camera owners can carry out the replacement of the screen themselves without too much trouble as long as the process is carried out carefully in the most dust-free environment available.
Replacement focusing screens are available from the following companies:

One problem, however, is that the screen and/or the mirror are often not perfectly aligned in many new cameras, and it only takes a minimal deviation (less than 1/100th to 1/10th of a millimeter) to produce a visible focus error (front focus or back focus). As long as the user continues to work with the AF system of the camera and default focusing screen, this minor error is generally not conspicuous. However, if the screen is replaced with a variant that includes focusing aids, then the deviation becomes visible.
The only action that can be taken in the face of this dilemma is to send the camera to the manufacturer or an authorized service center and request that they align the screen properly. For many camera models, aligning the focusing screen involves inserting or removing the corresponding number of thin layers of film.

Electronic focusing aids: fast, but not always perfect

In most of the Canon EOS, Nikon AF and Pentax AF cameras, whether digital or analog, electronic focus confirmation continues to be available even when using our ZE, ZF or ZK lenses. The camera confirms that the manual focus is correct by illuminating the focus confirmation light or the active AF point. In many of the Nikon models (e.g. the D700 and D3 series), additional support is provided by two arrows in the viewfinder which indicate the direction of rotation for the focus ring when performing fast manual focusing.
However, these electronic focusing aids only actually provide a relatively imprecise means of achieving high-precision manual focusing. The region shown as "in focus" when rotating the focus ring is generally quite large and is also dependent on the direction from which the subject is being brought into focus (i.e. whether you are coming from infinity or from the closest focus distance). We therefore recommend comprehensively testing the camera in combination with a manual lens in order to get a photographer's feel for the situations in which you can rely on the AF indicator. Especially when using fast lenses, it is advisable to take a bracketing series with a wide-open aperture and short shooting distances in order to achieve optimum results.
It is sometimes possible to fall wide of the mark when taking shots with the aid of the focus confirmation function, but fortunately some recent camera models (e.g. the Nikon D300, D700, D3 series and the Canon EOS 5D MkII) have incorporated an AF fine-tuning option in the menu which obviously also takes effect on the focus indicator during manual focusing. If the AF indicator consistently gives an incorrect response when a certain type of lens is used or even with all lenses, then the focus error can generally be rectified by carrying out meticulous checks and adjustments.

Live View mode

Many of the latest DSLR cameras enable a live image to be viewed on the display prior to taking the shot. It is usually possible to zoom into this view far enough to carry out focusing with great precision. Since the camera's mirror has to be locked up in this mode, the AF function is either no longer available or the mirror has to be briefly flipped down and then back up again to carry out focusing, depending on the specific camera system. A further option is the rather cumbersome contrast-based autofocus function used in compact cameras.
With the camera mounted on a tripod and zoomed into the Live View, a high-precision MF lens enables accurate focusing. In addition, Live View generally provides a bright, crystal-clear image even in poor light.
Alignment tolerances of the focusing screen, the mirror or the entire AF system of the camera are no longer an issue if Live View is used for manual focusing. At least in those cameras that take the preview image directly from the image sensor, you see the allocation of the focus exactly as it will subsequently appear in the shot.

Focus shift

Yet even with a perfectly aligned focusing screen with focusing aids, autofocus that is cleanly aligned or adjusted using the camera's menu and the Live View function, it is impossible to guarantee proper checking of the focus prior to shooting in every single case.
Spherical aberration in lenses leads to so-called focus shifts. This term refers to the fact that changing the aperture causes a shift in the optimum focus position. This effect is particularly conspicuous in the case of fast prime lenses. You can find a more detailed description of focus shift in the article "Measuring lenses objectively" which appears in Camera Lens News Nr. 30 starting on page 24.

In order to take into account the effect of focus shift on precise focusing, the user should – as far as possible – carry out focusing at the aperture at which the shot is subsequently to be taken. Nevertheless, there are still a number of limitations to be considered:
AF systems in current camera models do not take into account the focus shift of the lens. This means that the reliability of the AF indicator varies depending on the type of lens, f-stops and shooting distances.

Due to their surface structure, focusing screens in fast lenses are not capable of factoring in all the incoming rays. This can easily be seen from the fact that – depending on the design – the brightness of the focusing screen no longer changes with lenses such as those that are faster than 1:2.8. This means that the change in the optimum focus position can no longer be evaluated on the focusing screen at wide apertures when using very fast lenses.

And there is also a “fly in the ointment” when it comes to the use of the Live View function, namely the fact that many current camera models (e.g. the Nikon D700 and the Canon EOS 5D MkII) automatically control the aperture in the Live View mode based on the set ISO values and the ambient light in order to ensure that the image in the display is consistently shown with the correct brightness and contrast values. This means that it is no longer possible to close the aperture to the desired, fixed setting using the preview button. Correct evaluation of the depth of field and focus position is therefore equally out of reach using this method, since the aperture is only closed to the desired setting when the shot is actually taken.

Conclusion

When your aim is to take photographs with wide apertures and extremely accurate focusing, high-precision manual lenses can make your work easier. Enhanced focusing screens, a well-aligned AF system and use of the Live View mode all help to make focusing more precise. Although the influence of focus shift cannot be ignored, it can produce better, reproducible results if suitable care is taken. In situations where focusing accuracy is of paramount importance, it is also advisable to take finely graded bracketing sequences.

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